A variety of signal transmitting devices include outputs that are, for the most part, digital, but also include an analog aspect due to particular signal wave shape requirements. An example of such a transmitting device is a process variable transmitter. Process variable transmitters are used in a variety of industrial applications, and provide an electrical output signal corresponding to a sensed condition signal generated by a process variable (e.g., temperature, pressure, pH, etc.) sensor. The electrical output signal of the sensor is translated into a corresponding measurement value for the particular detected environmental variable type. The corresponding measurement value, in turn, is converted into an output signal that is transmitted to any of a variety of recipients, including a process controller. The process controller performs some action regarding the received output signal. At one point in time, such output signal was provided almost exclusively in the form of a 4–20 milliamp current signal. However, today digital communication standards exist and have substantially grown in use within industrial process control applications. It is now common for process control data, including process state information provided by process variable transmitters, to be transmitted in both a digital form superimposed as an AC signal on the 4–20 milliamp DC signal.
Digital signals are generally considered to have less exacting standards with regard to wave shape. The data exists within steady-state signal levels, frequencies, and/or phases. However, rather than requiring an exact value, each value must fall within specified ranges around signal levels/frequencies/phases. For example binary digital signals (having only two possible logical values) have defined high and low signal levels—with a buffer region separating these two levels. As a result, simple, inexpensive digital output circuits are known that provide minimized rise and fall times.
However, if not properly formed, digital signals can become a source of interference. One such form of interference that arises when data is output in the form of a square wave is cross talk. Such problem is well documented in the prior art. One way to suppress cross talk interference is to wave shape the output to reduce/eliminate abrupt signal level rate/slope changes—which increase the incidence of cross talk. If several wires are bundled together in close proximity to one another, interference/cross-talk becomes a substantial digital signal quality issue. As a result, even digital signal communications standards include specifications for rising/falling edges to limit interference/cross-talk and ensure proper operation of physical communication links embodying the communications standards. In such instances, the digital data signal waveforms actually have analog aspects that reduce the level of interference (e.g., cross talk) generated by the digital signal waveforms. By way of example, the transition points of a square wave are controlled (e.g. curved) to reduce/eliminate sharp transitions.
Transmitting control and data information between components of an industrial process control system is governed by communication protocols. Such communication protocols specify standards for transmitting waveforms to ensure proper operation of the devices that communicate via the protocols, and more particularly, ensure proper interpretation of received signal transmissions. By way of example, a well-known communication protocol in the industrial process control industry, known as the HART (Highway Addressable Remote Transducer) protocol/communications standard, includes a physical layer specification requiring a particular shape for the rising and falling edges of a waveform. In particular, the HART protocol requires a linear ramp (up/down), having a specified duration, between high and low physical signal levels.
A simple resistor/capacitor output stage, driven by a simple high or low input square wave signal, is inappropriate for HART devices. Such output stage generates an exponential rise and fall that does not meet signal level transition characteristics (i.e., approximating a linear ramp having particular rise and fall times) required by the HART protocol physical layer specification. In such instances complex analog circuitry, such as one or more current sources coupled to one or more resistor/capacitor filter stages, is required to meet the exacting signal transition specifications of the HART protocol. Yet another potential technique for generating an analog output of arbitrary shape involves using a digital-to-analog converter that receives a stream of digital values controlling an analog output of a specific shape. The analog output of the digital-to-analog converter provides an output that closely follows the present digital input code in order to produce both steady state signals as well as shape signal transitions between the steady states.
The above example, addressing requirements of the HART protocol, is only one of many potential instances where a largely digital communications protocol incorporates analog signal standards—such as for example requiring a time-varying output signal to conform to a specific shape—that, in-turn, requires incorporation of output circuitry capable of producing a signal having particular wave shape properties.